Transcript Lecture 1: Overview - City University of New York

Lecture 5:
Uniprocessor Scheduling
Advanced Operating System
Fall 2010
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Contents
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Basic Concepts
Scheduling Criteria
Scheduling Algorithms
Uniprocessor scheduling – multilevel
queue scheduling
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Basic Concepts
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Scheduling is concerned with the decision of assigning processor
(or processors) to processes in such a way so as to optimize a
number of performance criteria. These performance criteria can
be user-oriented or system-oriented.
Types of Scheduling
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Long-term scheduling – the decision to add to the pool of
processes to be executed.
Medium-term scheduling – The decision to add to the number of
processes that are partially or full in main memory.
Short-term scheduling – The decision as to which available process
will be executed by the processor.
I/O scheduling – The decision as to which process’ pending I/O
request shall be handled by the available I/O device.
The first 3 types of scheduling are processor scheduling. The last
one is a function of the characteristic of the I/O device – e.g. disk
or tape.
We shall discuss about the first 3 types.
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Long-Term Scheduling
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The long-term scheduling determines which programs are
admitted to the system for processing. Thus, it controls the
degree of multiprogramming.
For interactive programs in a time-sharing system, a process
request can be generated by the act of a user attempting to
connect to the system. The OS will accept all authorized users
until the system is saturated, using some predefined measure
of saturation. Once the predefined measure of saturation is
reached, the next request will be blocked with a message that
the system is full and the user should try again later.
In a batch system, a newly submitted job is first routed to a
disk. The long-term scheduler decides when to admit the new
job into the system. There are 2 decisions involved here:
1.
2.
The long-term scheduler must decide when to take on an
additional job.
Which job (or jobs) to choose.
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Long-Term Scheduling (cont.)
1.
2.
For decision 1. the decision is driven by the desired
degree of multiprogramming. The degree of
multiprogramming is dictated by the available
resources (e.g. enough memory.
For decision 2, the decision can be on a FCFS policy,
or certain criteria based on priority, expected
execution time, or I/O requirements. For example,
the scheduler may attempt to keep a mix of
processor-bound and I/O-bound processes. The
scheduler may attempt to keep a good mix of short
jobs (student jobs) and long jobs (research
projects)
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Medium-term scheduling
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Medium-term scheduling is part of the swap-out
function. Typically, blocked processes are the ones to
swap-out. It is a function of the desired degree of
multiprogramming as well as the cost of swapping.
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Short-term scheduling
(dispatcher)
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The short-term scheduler is invoked
whenever an event occurs that may lead to
the suspension of the current process or that
may provide an opportunity to preempt a
currently running process in favor of another.
Examples of such events include the following:
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Clock interrupts
I/O interrupts
OS calls
Signals
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Dispatcher
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Dispatcher module gives control of the CPU
to the process selected by the short-term
scheduler; this involves:
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switching context
switching to user mode
jumping to the proper location in the user
program to restart that program
Dispatch latency – time it takes for the
dispatcher to stop one process and start
another running
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Short-term Scheduling Criteria
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System-Oriented Criteria
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CPU utilization – percentage time that the processor is busy, keep
the CPU as busy as possible (maximize processor utilization)
Throughput – # of processes that complete their execution per
time unit (maximize throughput)
User-Oriented Criteria
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Turnaround time – interval of time between submission of a job
and its completion, i.e., the total time that the process spends in
the system (waiting time plus service time). Appropriate measure
for batch job.
Waiting time – amount of time a process has been waiting in the
ready queue
Response time – amount of time it takes from when a request
was submitted until the first response is produced, not output
(for time-sharing environment)
Deadline – The ability to meet hard deadline. Appropriate for realtime jobs
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Short-term Scheduling
CPU Scheduler
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Selects from among the processes in memory that
are ready to execute, and allocates the CPU to one of
them
Scheduling Discipline:
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Nonpreemptive – Once a process is in the Running State, it
continues to execute until
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Preemptive – The currently running process may be
interrupted and moved to the Ready State by the OS
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(a) it terminates
(b) blocks itself to wait for I/O or to request some OS services
(a) Switches from running to ready state
(b) Switches from waiting to ready state
Preemptive more overhead than nonpreemptive, but
may provide better service
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Optimization Criteria
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Max CPU utilization
Max throughput
Min turnaround time
Min waiting time
Min response time
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Scheduling Algorithm
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Turnaround Time(TAT) is the total time
that the process spends in the system
(waiting time + service time).
Normalized Turnaround Time is the
ratio of Turnaround Time to Service
Time
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First-Come, First-Served (FCFS)
Scheduling
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Process Arrival Time Service Time
A
0
3
B
2
6
C
4
4
D
6
5
E
8
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The Gantt Chart for the schedule is:
A
0
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B
3
D
C
9
13
E
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Turnaround Time(TAT) A: 3; B: 7; C: 9; D: 12; E: 12
Normalized Turnaround Time: A: 3/3=1.0; B: 7/6; C:
9/4=2.25; D: 12/5=2.4; E: 12/2=6.0
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Round Robin (RR)
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Used in interactive time-sharing system.
Each process gets a small unit of CPU time (time quantum),
usually 10-100 milliseconds. After this time has elapsed, the
process is preempted and added to the end of the ready
queue.
If there are n processes in the ready queue and the time
quantum is q, then each process gets 1/n of the CPU time in
chunks of at most q time units at once. No process waits
more than (n-1)q time units.
Performance
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q large  FIFO
q small  processor sharing
q must be large with respect to context switch, otherwise
overhead is too high
Best choice q for will be slightly greater than the time required
for a typical interaction.
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Example of RR with Time
Quantum = 1
Process
Arrival Time
A
0
B
2
C
4
D
6
E
8
The Gantt chart is:
A
0
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A
1
A
2
B
3
B
4
C
5
Service Time
3
6
4
5
2
B
6
C D
7
8
B C
E
D
B
C D
E
B
D
9 10 11 12 13 14 15 16 17 18 19
Turnaround Time(TAT) A: 3; B: 16; C: 11; D: 14; E: 10
Normalized Turnaround Time: A: 3/3=1.0; B: 16/6=8/3=2.66; C:
11/4=2.75; D: 14/5=2.8; E: 10/2=5
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Time Quantum and Context Switch Time
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Shortest- Process-Next (SPN)
Scheduling
This is a nonpreemptive scheduling discipline in which the shortest expected
processing time is selected next.
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Process
Arrival Time
A
0
3
B
2
6
C
4
4
D
6
5
E
8
2
The Gantt chart is:
A
0
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B
3
Service Time
C
E
9
11
D
15
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Turnaround Time(TAT) A: 3; B: 7; E: 3; C: 11; D: 14
Normalized Turnaround Time: A: 3/3=1; B: 7/6; E: 3/2; C: 11/4; D: 14/5
Needs user to provide an estimate of processing time.
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Determining Length of Next CPU
Burst (not required)
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For interactive processes, the OS may keep a running average of each “burst” for each
process and estimate the processing time requirement
of the next burst by
n
S n 1 
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1
n
T
i 1
i
Ti= processor execution time for the ith instance of this process
Si= predicted value for the ith instance
S1=predicted value for the 1st instance, not calculated, assumed to be 0.
The equation can be rewritten as
1
n 1
S n 1  Tn 
Sn
n
n
The above formular gives equal weight to each instance. If we want to give greater
weight to more recent instances, then we can use an exponential averaging
Sn1  Tn  (1   )Sn
where α is a constant and 0< α <1,
Sn1  Tn  (1   )Tn1  (1   )2Tn2  ... (1   )n1T1
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Because both α and (1- α) are less than 1, each successive term in the preceding
equation is smalle.
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The larger the value of α, the greater the weight given to the more recent observations.
Shortest Remaining Time(SRT)
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It is a preemptive version of SPN. The scheduler always chooses the process that has the
shortest expected remaining processing time.
Process
Arrival Time
A
0
B
2
C
4
D
6
E
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The Gantt chart is:
A
0
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B
3 4
C
Service Time
3
6
4
5
2
E
8
B
10
D
15
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Turnaround Time(TAT) A: 3; B: 13; C: 4; D: 14; E: 2
Normalized Turnaround Time: A: 3/3=1; B: 13/6; C: 4/4=1; D: 14/5=2.8; E:
2/2=1
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Priority Scheduling
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A priority number (integer) is associated with each process
The CPU is allocated to the process with the highest priority
(smallest integer  highest priority)
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Preemptive
nonpreemptive
SJF is a priority scheduling where priority is the predicted next
CPU burst time
FCFS is a priority scheduling where priority is the arrival time of
the process.
Problem  Starvation – low priority processes may never
execute
Solution  Aging – as time progresses increase the priority of
the process
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Highest Response Ratio Next
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w= time spent waiting for the processor
s= expected service time
R=(w+s)/s
When a process is free for assignment, choose the process with the greatest value of R.
Process
Arrival Time
A
0
B
2
C
4
D
6
E
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The Gantt chart is:
A
0
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3
6
4
5
2
B
3
Service Time
E
C
9
13
D
15
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Calculate R:
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at time 9: C: ((9-4)+4)/4=9/4; D: ((9-6)+5)/5=8/5; E: ((9-8)+2)/2=3/2, choose C
At time 13: D: ((13-6)+5)/5=2.4; E: ((13-8)+2)/2=3.5; choose E.
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Multilevel Queue
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Ready queue is partitioned into separate queues:
foreground (interactive)
background (batch)
Each queue has its own scheduling algorithm
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foreground – RR
background – FCFS
Scheduling must be done between the queues
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Fixed priority scheduling; (i.e., serve all from foreground
then from background). Possibility of starvation.
Time slice – each queue gets a certain amount of CPU time
which it can schedule amongst its processes; i.e., 80% to
foreground in RR, 20% to background in FCFS
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Multilevel Queue Scheduling
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Multilevel Feedback Queue
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A process can move between the various queues;
aging can be implemented this way
Multilevel-feedback-queue scheduler defined by
the following parameters:
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number of queues
scheduling algorithms for each queue
method used to determine when to upgrade a process
method used to determine when to demote a process
method used to determine which queue a process will
enter when that process needs service
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Example of Multilevel Feedback Queue
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Three queues:
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Q0 – RR with time quantum 8 milliseconds
Q1 – RR time quantum 16 milliseconds
Q2 – FCFS
Scheduling
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A new job enters queue Q0 which is served FCFS. When it gains CPU, job receives 8
milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1.
At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does
not complete, it is preempted and moved to queue Q2.
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End of lecture 5
Thank you!
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